Thermal spraying may be a promising approach for the deposition of NiAl-Al 2 O 3 composite coatings. To understand the intrinsic mechanical property of NiAl-Al 2 O 3 composite, dense NiAl-Al 2 O 3 composites were prepared by plasma activated sintering using the ball milled Ni/Al- Al 2 O 3 composite powders contain both micro-sized Al 2 O 3 strengthening particles and submicron-sized Al 2 O 3 dispersoids homogeneously distributed in the NiAl matrix phase. The angular morphology of the Al 2 O 3 particles in composite powder was changed to spherical or near spherical morphology after plasma activated sintering. Compared to the NiAl-Al 2 O 3 composites with low Al 2 O 3 content, NiAl- 60vol.%Al 2 O 3 composite consisted of less fine submicron-sized Al 2 O 3 dispersoids. Melting induced mechanism was proposed to explain the spheroidization and the change of Al 2 O 3 particle size distribution. The hardness of the NiAl-Al 2 O 3 composites with both micro-sized Al 2 O 3 strengthening particles and submicron-sized Al 2 O 3 dispersoids increased with the increase of Al 2 O 3 content. The relation was employed to theoretically estimate the hardness using volume fractions of components and corresponding hardness. The estimated hardness was compared with the observed ones. It was found that bimodal sized particles reinforcement results in a higher hardness than the theoretical value, which could be attributed to the dispersion hardening effect.